1. Field of the Invention
The present invention relates to wastewater treatment methods and wastewater treatment apparatuses and, in particular, to a wastewater treatment method and a wastewater treatment apparatus in which a flocculant and magnetic particles are added to wastewater to treat the wastewater with flocculation magnetic separation.
2. Description of the Related Art
A water treatment method with a flocculation magnetic separation system is the system in which a flocculant and magnetic particles are added to wastewater to be treated, flocculation flocs (hereinafter referred to as magnetic flocs) formed by stirring the wastewater are collected with magnetic force of a magnetic separation apparatus, and thereby treated water is obtained.
With this system, it is necessary to dispose of collected magnetic flocs as industrial waste although the magnetic flocs containing the magnetic particles are collected, and thus supply cost of the magnetic particles and disposal cost of the collected flocs as the industrial waste have caused the increase of running cost.
As a technology for solving this problem, disclosed in Japanese Patent Application Laid-Open No. 11-123399 and Japanese Patent Application Laid-Open No. 11-207399 is the technology in which sludge (magnetic flocs) containing magnetic particles is decomposed with a hydrothermal reaction to thereby reduce the sludge.
Wastewater treatment apparatuses disclosed in Japanese Patent Application Laid-Open No. 11-123399 and Japanese Patent Application Laid-Open No. 11-207399 are the apparatuses in which dirty water is purified with flocculation magnetic separation by using magnetic separation apparatuses, sludge generated at the time is hydrothermally treated under high temperature and pressure, and magnetic particles are collected with magnetic separation in high temperature and pressure lines.
In flocculation processes of wastewater in the wastewater treatment apparatuses disclosed in Japanese Patent Application Laid-Open No. 11-123399 and Japanese Patent Application Laid-Open No. 11-207399, an inorganic flocculant is first added to the wastewater together with magnetic particles, microflocs are formed by rapidly stirring the wastewater, and subsequently, magnetic flocs are made to grow larger by adding a polymeric flocculant to the wastewater and then stirring it slowly. Namely, first, surface charges of solids in the water are neutralized with the inorganic flocculant to make the solids easy to flocculate, a contact frequency of each particle is increased by rapidly stirring the wastewater to thereby promote flocculation, and formed are small magnetic flocs in which the magnetic particles and the solids are uniformly mixed. Since there is much inorganic flocculant on surfaces of the magnetic flocs in this state, many of them are positively charged.
Consequently, the respective magnetic flocs are got together to form huge magnetic flocs by adding an anionic (negatively charged) polymeric flocculant, and improved is separation performance in a magnetic separation process of a subsequent stage. Here, a stirring intensity is reduced after adding the polymeric flocculant to the wastewater, which is because cross-links with the polymeric flocculant are destroyed as little as possible. In the magnetic flocs formed as described above, the solids and the magnetic particles to be removed are particularly firmly combined with each other with the inorganic flocculant, thus requiring a floc decomposition process with a hydrothermal reaction in order to decompose the combinations.
However, in the wastewater treatment apparatuses disclosed in Japanese Patent Application Laid-Open No. 11-123399 and Japanese Patent Application Laid-Open No. 11-207399, since the solids and the magnetic particles are firmly combined with each other with the inorganic flocculant, considerable energy must be given to obtain sufficient floc decomposition performance, and a structure of a hydrothermal reaction apparatus as a floc decomposition process also becomes complex.
According to Japanese Patent Application Laid-Open No. 11-123399, it is disclosed that sludge is guided to a magnetic particle separation apparatus after pressurizing it to a pressure not less than 10 MPa and heating it to a temperature less than approximately 350° C. In addition, according to Japanese Patent Application Laid-Open No. 11-207399, it is disclosed that sludge is guided to a magnetic particle separation apparatus after pressurizing it to a pressure approximately 2 MPa and heating it to a temperature around 200° C.